kernel/thread: Must always initialize is_idle field

Our thread struct gets initialized piecewise in a bunch of locations
(this is sort of a design flaw).  The is_idle field, which was
introduced to identify idle threads in SMP (where there can be more
than one), was correctly set for idle threads but was being left
uninitialized elsewhere, and in a tiny handful of cases was turning up
nonzero.

The case in pipes. was particularly vexsome, as that isn't a thread at
all but one of the "dummy" threads used for timeouts (another design
flaw IMHO).

Get this right everywhere.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>